US20050085894A1 - High strength and lubricious materials for vascular grafts - Google Patents

Abstract

A stent-graft fabricated from a high strength and lubricious material provides for a more durable and lower profile endoprosthesis. The stent-graft comprises one or more stent segments covered with a fabric formed by the weaving, knitting or braiding of a biocompatible, high tensile strength, abrasion resistant, highly durable fiber such as spider dragline silk. The one or more stent segments may be balloon expandable or self-expanding. The fabric may be attached to the stent segments utilizing any number of known materials and techniques.

Description

BACKGROUND OF THE INVENTION
• 1. Field of the Invention
• The present invention relates to devices and methods for repairing aneurysms and more particularly, to percutaneously and/or intraluminally delivered devices and methods for repairing aneurysms such as abdominal aortic aneurysms and thoracic aortic aneurysms. The present invention also relates to materials for fabricating grafts.
• 2. Discussion of the Related Art
• An aneurysm is an abnormal dilation of a layer or layers of an arterial wall, usually caused by a systemic collagen synthetic or structural defect. An abdominal aortic aneurysm is an aneurysm in the abdominal portion of the aorta, usually located in or near one or both of the two iliac arteries or near the renal arteries. The aneurysm often arises in the infrarenal portion of the diseased aorta, for example, below the kidneys. A thoracic aortic aneurysm is an aneurysm in the thoracic portion of the aorta. When left untreated, the aneurysm may rupture, usually causing rapid fatal hemorrhaging.
• Aneurysms may be classified or typed by their position as well as by the number of aneurysms in a cluster. Typically, abdominal aortic aneurysms may be classified into five types. A Type I aneurysm is a single dilation located between the renal arteries and the iliac arteries. Typically, in a Type I aneurysm, the aorta is healthy between the renal arteries and the aneurysm and between the aneurysm and the iliac arteries.
• A Type II A aneurysm is a single dilation located between the renal arteries and the iliac arteries. In a Type II A aneurysm, the aorta is healthy between the renal arteries and the aneurysm, but not healthy between the aneurysm and the iliac arteries. In other words, the dilation extends to the aortic bifurcation. A Type II B aneurysm comprises three dilations. One dilation is located between the renal arteries and the iliac arteries. Like a Type II A aneurysm, the aorta is healthy between the aneurysm and the renal arteries, but not healthy between the aneurysm and the iliac arteries. The other two dilations are located in the iliac arteries between the aortic bifurcation and the bifurcations between the external iliacs and the internal iliacs. The iliac arteries are healthy between the iliac bifurcation and the aneurysms. A Type II C aneurysm also comprises three dilations. However, in a Type II C aneurysm, the dilations in the iliac arteries extend to the iliac bifurcation.
• A Type III aneurysm is a single dilation located between the renal arteries and the iliac arteries. In a Type III aneurysm, the aorta is not healthy between the renal arteries and the aneurysm. In other words, the dilation extends to the renal arteries.
• A ruptured abdominal aortic aneurysm is presently the thirteenth leading cause of death in the United States. The routine management of abdominal aortic aneurysms has been surgical bypass, with the placement of a graft in the involved or dilated segment. Although resection with a synthetic graft via transperitoneal or retroperitoneal procedure has been the standard treatment, it is associated with significant risk. For example, complications include perioperative myocardial ischemia, renal failure, erectile impotence, intestinal ischemia, infection, lower limb ischemia, spinal cord injury with paralysis, aorta-enteric fistula, and death. Surgical treatment of abdominal aortic aneurysms is associated with an overall mortality rate of five percent in asymptomatic patients, sixteen to nineteen percent in symptomatic patients, and is as high as fifty percent in patients with ruptured abdominal aortic aneurysms.
• Disadvantages associated with conventional surgery, in addition to the high mortality rate, include an extended recovery period associated with the large surgical incision and the opening of the abdominal cavity, difficulties in suturing the graft to the aorta, the loss of the existing thrombosis to support and reinforce the graft, the unsuitability of the surgery for many patients having abdominal aortic aneurysms, and the problems associated with performing the surgery on an emergency basis after the aneurysm has ruptured. Further, the typical recovery period is from one to two weeks in the hospital and a convalescence period, at home, ranging from two to three months or more, if complications ensue. Since many patients having abdominal aortic aneurysms have other chronic illnesses, such as heart, lung, liver and/or kidney disease, coupled with the fact that many of these patients are older, they are less than ideal candidates for surgery.
• The occurrence of aneurysms is not confined to the abdominal region. While abdominal aortic aneurysms are generally the most common, aneurysms in other regions of the aorta or one of its branches are possible. For example, aneurysms may occur in the thoracic aorta. As is the case with abdominal aortic aneurysms, the widely accepted approach to treating an aneurysm in the thoracic aorta is surgical repair, involving replacing the aneurysmal segment with a prosthetic device. This surgery, as described above, is a major undertaking, with associated high risks and with significant mortality and morbidity.
• Over the past five years, there has been a great deal of research directed at developing less invasive, endovascular, i.e., catheter directed, techniques for the treatment of aneurysms, specifically abdominal aortic aneurysms. This has been facilitated by the development of vascular stents, which can and have been used in conjunction with standard or thin-wall graft material in order to create a stent-graft or endograft. The potential advantages of less invasive treatments have included reduced surgical morbidity and mortality along with shorter hospital and intensive care unit stays.
• Stent-grafts or endoprostheses are now Food and Drug Administration (FDA) approved and commercially available. Their delivery procedure typically involves advanced angiographic techniques performed through vascular accesses gained via surgical cutdown of a remote artery, which may include the common femoral or brachial arteries. Over a guidewire, the appropriate size introducer will be placed. The catheter and guidewire are passed through the aneurysm. Through the introducer, the stent-graft will be advanced to the appropriate position. Typical deployment of the stent-graft device requires withdrawal of an outer sheath while maintaining the position of the stent-graft with an inner-stabilizing device. Most stent-grafts are self-expanding; however, an additional angioplasty procedure, e.g., balloon angioplasty, may be required to secure the position of the stent-graft. Following the placement of the stent-graft, standard angiographic views may be obtained.
• Due to the large diameter of the above-described devices, typically greater than twenty French (3F=1 mm), arteriotomy closure typically requires open surgical repair. Some procedures may require additional surgical techniques, such as hypogastric artery embolization, vessel ligation, or surgical bypass in order to adequately treat the aneurysm or to maintain blood flow to both lower extremities. Likewise, some procedures will require additional advanced catheter directed techniques, such as angioplasty, stent placement and embolization, in order to successfully exclude the aneurysm and efficiently manage leaks.
• While the above-described endoprostheses represent a significant improvement over conventional surgical techniques, there is a need to improve the endoprostheses, their method of use and their applicability to varied biological conditions. Accordingly, in order to provide a safe and effective alternate means for treating aneurysms, including abdominal aortic aneurysms and thoracic aortic aneurysms, a number of difficulties associated with currently known endoprostheses and their delivery systems must be overcome. One concern with the use of endoprostheses is the prevention of endo-leaks and the disruption of the normal fluid dynamics of the vasculature. Devices using any technology should preferably be simple to position and reposition as necessary, should preferably provide an acute, fluid tight seal, and should preferably be anchored to prevent migration without interfering with normal blood flow in both the aneurysmal vessel as well as branching vessels. In addition, devices using the technology should preferably be able to be anchored, sealed, and maintained in bifurcated vessels, tortuous vessels, highly angulated vessels, partially diseased vessels, calcified vessels, odd shaped vessels, short vessels, and long vessels. In order to accomplish this, the endoprostheses should preferably be highly durable, extendable and re-configurable while maintaining acute and long-term fluid tight seals and anchoring positions.
• The endoprostheses should also preferably be able to be delivered percutaneously utilizing catheters, guidewires and other devices which substantially eliminate the need for open surgical intervention. Accordingly, the diameter of the endoprostheses in the catheter is an important factor. This is especially true for aneurysms in the larger vessels, such as the thoracic aorta.
• One component of the endoprosthesis that affects the size, seal and durability is the graft material. In any endoprosthesis design, it would be preferable that the graft material be constructed or fabricated from fibers that are thin, strong, durable and lubricious.
SUMMARY OF THE INVENTION
• The present invention overcomes the potential disadvantages associated with percutaneously delivered endoprostheses as briefly described above.
• In accordance with one aspect, the present invention is directed to an endovascular graft. The endovascular graft comprises one or more scaffold structures, a biocompatible, high tensile strength, abrasion resistant, highly durable thin-walled graft material affixed to the one or more scaffold structures, and at least one connector for connecting the graft material to the one or more scaffold structures. The graft material including spider dragline silk.
• The abrasion resistant stent-graft of the present invention comprises at least one stent segment and a highly durable, abrasion-resistant graft material attached thereto. The graft material may be attached to the at least one stent segment in any number of ways. The stent-graft may be utilized as a component of a larger system, for example, in a system for repairing abdominal aortic aneurysms, or as a stand-alone device. In either embodiment, the stent-graft is utilized as a fluid carrying conduit that is preferably percutaneously delivered, but may also be utilized surgically. The at least one stent segment may comprise any suitable scaffold structure and may be fabricated from any number of biocompatible materials. The at least one stent segment may be self-expanding or balloon expandable.
• The abrasion resistant stent-graft of the present invention is preferably percutaneously delivered, and as such it is preferably designed with the smallest diameter possible. In order to achieve the smallest diameter possible, thinner graft materials are needed. However, stent-grafts are typically positioned within the body in vessels that have relatively high hydrodynamic forces, thus requiring graft materials which are able to withstand these forces. Essentially, these forces tend to wear the graft material at the points where it is connected to the at least one stent segment. Over time, the graft material may develop microleaks which obviously defeat the purpose of the stent-graft, namely, as a by-pass conduit. Accordingly, the abrasion resistant stent-graft of the present invention utilizes a biocompatible, high tensile strength, abrasion resistant, highly durable yarn which may be woven, knitted or braided into a graft material without sacrificing diameter.
• The yarn, fibers or thread may comprise a single component or it may be blended with one or more other suitable materials to achieve various desirable characteristics, including abrasion resistance, flexibility and thinness. One such yarn comprises ultra high molecular weight polyethylene, which is commercially available. Accordingly, the abrasion resistant stent-graft of the present invention is a highly durable stent-graft which, because of its thin graft material, may be percutaneously delivered more easily than present stent-grafts.
• The yarn, fibers or thread may comprise proteins such as silk. The strength, toughness and elasticity of silk is widely known. The dragline silk of spiders is the fiber from which spiders make the scaffolding of their webs. Given its strength and elasticity, it may be utilized in the construction of ultra-thin walled stent-grafts or covered stents.
BRIEF DESCRIPTION OF THE DRAWINGS
• The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.
• FIG. 1 is an elevational view of an endovascular graft in accordance with the present invention.
• FIG. 2 is a perspective view of an expanded stent segment of the endovascular graft in accordance with the present invention.
• FIG. 2A is a fragmentary perspective view of a portion of the stent segment ofFIG. 2.
• FIG. 2B is a fragmentary perspective view of a portion of the stent segment ofFIG. 2.
• FIG. 2C is an enlarged plan view of a section of the stent segment ofFIG. 2.
• FIG. 2D is an enlarged plan view of a section of the stent segment ofFIG. 2.
• FIG. 3 is a perspective view of another expanded stent segment of the endovascular graft in accordance with the present invention.
• FIG. 4 is an elevational view of an endovascular graft in accordance with the present invention.
• FIG. 5 is a diagrammatic representation of a graft comprising spider dragline silk in accordance with the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
• The present invention is directed to an endovascular graft which may be utilized as a component in a system for use in treating or repairing aneurysms. Systems for treating or repairing aneurysms such as abdominal aortic aneurysms and thoracic aortic aneurysms come in many forms. A typical system includes an anchoring and/or sealing component which is positioned in healthy tissue above the aneurysm and one or more grafts which are in fluid communication with the anchoring and/or sealing component and extend through the aneurysm and anchor in healthy tissue below the aneurysm. Essentially, the grafts are the components of the system that are utilized to establish a fluid flow path from one section of an artery to another section of the same or different artery, thereby bypassing the diseased portion of the artery. Current systems are preferably percutaneously delivered and deployed.
• As stated above, the present invention is directed to one component of an aneurysm repair system; namely, the endovascular graft of stent-graft. Accordingly, the following detailed description is directed to the endovascular graft. The endovascular graft comprises at least one stent segment and a highly durable, abrasion-resistant graft material attached thereto. In other words, the endovascular graft of the present invention is supported internally by one or more individual stents, which are themselves connected to the graft in a manner which secures their position, for example, by sutures. It is important to note that while one particular stent design is discussed in detail below, the graft of the present invention may incorporate any number of suitable stent designs, including self-expanding stents and balloon expandable stents. In addition, the endovascular graft may comprise a device formed solely from the graft material.
• FIG. 1 illustrates an exemplary embodiment of anendovascular graft10 in accordance with the present invention. The exemplaryendovascular graft10 comprises one or morefirst stent segments100, onesecond stent segment200 and athird stent segment300. In order to illustrate the relationship of the various components comprising theendovascular graft10, the endovascular graft is illustrated in the figure as though the graft material were transparent. In a typical use scenario, thethird stent segment300 would be anchored in healthy tissue below the aneurysm and the uppermostfirst stent segment100 would be in fluid communication with an anchoring and/or sealing component as briefly described above. It is important to note, however, that depending on the design of the system, an anchoring and/or sealing component may not be necessary. Thesecond stent segment200 comprises a tapered profile, having a diameter at one end equal to that of thefirst stent segments100 and a diameter at the other end equal to that of thethird stent segment300. The length of the endovascular graft may be varied by the number offirst stent segments100 utilized.
• FIG. 2 is a detailed perspective view of an exemplary embodiment of thethird stent segment300. Thethird stent segment300 comprises a plurality ofstruts302 connected in a substantially zigzag pattern. As illustrated, the. exemplarythird stent segment300 comprises three sets of zigzag-connectedstents302, thereby forming substantially diamond-shaped cells. Thenon-connected apex304 of each diamond shaped cell, illustrated in greater detail inFIG. 2A, comprises a smooth, uniform width curved region formed at the intersection of twostents302 of each diamond-shaped cell. This shape is cut directly into thestent segment300 during the initial machining steps, typically laser cutting, as is explained in detail subsequently, and is maintained during all subsequent finishing processing. Thejunctions306 between the zigzag-connectedstents302, illustrated in greater detail inFIG. 2B occurs at the intersection of four struts302. Preferably, eachjunction306 of fourstruts302 comprises twoindentations308 and310 as illustrated inFIG. 2B.
• The regions proximate thenon-connected apexes304 and thejunctions306 are generally the highest stress regions in thethird stent segment300. To minimize the stresses in these regions, these regions are designed to maintain uniform beam widths proximate where thestruts302 interconnect. Beam width refers to the width of astrut306.Indentations308 and310 are cut or machined into thejunctions306 to maintain a uniform beam width in this area, which is generally subject to the highest stress. Essentially, by designing thejunctions306 to maintain uniform beam widths, the stress and strain that would normally build up in a concentrated area, proximate thejunction306, is allowed to spread out into the connecting regions, thereby lowering the peak values of the stress and strain in the stent structure.
• To further minimize the maximum stresses in thestruts302 of thethird stent segment300, thestruts302 may have a tapering width. For example, in one exemplary embodiment, thestruts302 may be designed to become wider as it approaches ajunction306.FIG. 2C is an enlarged partial view of thethird sent segment300 in its expanded conditions which illustrates the tapering width of thestruts302. In this exemplary embodiment, thestrut302 proximate the junction306 (width a) is about 0.025 cm and gradually tapers to a dimension of about 0.0178 cm in the mid-region of the strut302 (width b). By tapering the struts' widths, the stresses in thestruts302 adjacent thejunction306 is spread out away from thejunction306. The tapering of thestruts302 is accomplished during the machining of the tube of material from which thestent300 is cut, as described in detail subsequently. However, by tapering thestruts302 in this manner, there is a tradeoff. Thestent segment300 becomes somewhat less resistant to localized deformations, caused for example, by a protrusion within the vessel lumen. This localized deformation may lead to a local torsional loading on some of thestruts302, and, therefore, since thestruts302 in this exemplary embodiment have a relatively significant portion of their length with a reduced width, their torsional rigidity is reduced.
• If maximizing the resistance to localized deformation is preferred, thestruts302 may be maintained at a uniform width, or more preferably have a reverse taper, as illustrated inFIG. 2D, wherein the width at point a is less than the width at point b. In this exemplary embodiment, the reverse taper struts302 are about 0.025 cm proximate thejunction306 and about 0.028 cm in the central region of the struts. While this reverse taper tends to increase the stresses somewhat proximate thejunctions306, this increase is very small relative to the decrease in stresses gained by having theside indentations308,310 illustrated inFIG. 2B, as well as the uniform width connections illustrated inFIG. 2A. In addition, since the reverse taper serves to increase the torsional rigidity of thestrut302, the stent structure resists local deformation and tends to maintain a substantially circular cross-sectional geometry, even if the lumen into which the stent is positioned in non-circular in cross-section.
• In a preferred exemplary embodiment, thethird stent segment300 is fabricated from a laser cut tube, as described in detail subsequently, of initial dimensions 0.229 cm inside diameter by 0.318 cm outside diameter. Thestruts302 are preferably 0.0229 cm wide adjacent the fourstrut junctions306 and six mm long, with a reverse taper strut width. Also, to minimize the number of different diameter combination of grafts systems, it is preferred that thethird stent segment300 have an expanded diameter of sixteen mm. Similarly, the proximal portion of the graft material forming the legs is flared, having a diameter of sixteen mm. This single diameter for the third stent segment of the graft system would enable its use in arteries having a non-aneurysmal region of a diameter from between eight and fourteen mm in diameter. It is also contemplated that multiple diameter combinations ofthird stent segment300 and graft flare would be desirable.
• Referring back toFIG. 1, the one or morefirst stent segments100 are also formed from a shape set laser cut tube, similar to thethird stent segment300 described above. The one or morefirst stent segments100 comprise a single circumferential row of zigzag or sinusoidally arranged elements. In the exemplary embodiment illustrated inFIG. 1, and in greater detail inFIG. 3, thefirst stent segment100 comprises ten zigzag or sinusoidal undulations. The one or morefirst stent segments100 are formed with uniform width connections at theintersections104 of thestruts102 forming the zigzag or sinusoidal pattern. The one or morefirst stent segments100 are preferably cut from tubing having an inside diameter of 0.251 cm and an outside diameter of 0.317 cm. The strut widths are preferably about 0.33 cm wideadjacent strut intersections104 and thestruts102 are preferably seven mm long and the one or morefirst stent segments100 are preferably eleven mm in diameter when expanded.
• Referring back toFIG. 1, thesecond stent segment200 comprises a tapered profile, having a diameter at one end which is the same as the one or morefirst stent segments100, and a diameter at the other end matching the diameter of thethird stent segment300. Thesecond stent segment200 is identical to the one or morefirst stent segments100 except for the taper.
• As is explained in detail subsequently, thestent segments100,200 and300 are secured in position by the graft material.
• The first, second andthird stent segments100,200,300 are preferably self-expandable and formed from a shape memory alloy. Such an alloy may be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The application of a desired temperature causes the alloy to revert to an original heat-stable configuration. A particularly preferred shape memory alloy for this application is binary nickel titanium alloy comprising about 55.8 percent Ni by weight, commercially available under the trade designation NITINOL. This NiTi alloy undergoes a phase transformation at physiological temperatures. A stent made of this material is deformable when chilled. Thus, at low temperatures, for example, below twenty degrees centigrade, the stent is compressed so that it can be delivered to the desired location. The stent may be kept at low temperatures by circulating chilled saline solutions. The stent expands when the chilled saline is removed and it is exposed to higher temperatures within the patient's body, generally around thirty-seven degrees centigrade.
• In preferred embodiments, each stent is fabricated from a single piece of alloy tubing. The tubing is laser cut, shape-set by placing the tubing on a mandrel, and heat-set to its desired expanded shape and size.
• In preferred embodiments, the shape setting is performed in stages at five hundred degrees centigrade. That is, the stents are placed on sequentially larger mandrels and briefly heated to five hundred degrees centigrade. To minimize grain growth, the total time of exposure to a temperature of five hundred degrees centigrade is limited to five minutes. The stents are given their final shape set for four minutes at five hundred fifty degrees centigrade, and then aged to a temperature of four hundred seventy degrees centigrade to import the proper martensite to austenite transformation temperature, then blasted, as described in detail subsequently, before electropolishing. This heat treatment process provides for a stent that has a martensite to austenite transformation which occurs over a relatively narrow temperature range; for example, around fifteen degrees centigrade.
• To improve the mechanical integrity of the stent, the rough edges left by the laser cutting are removed by combination of mechanical grit blasting and electropolishing. The grit blasting is performed to remove the brittle recast layer left by the laser cutting process. This layer is not readily removable by the electropolishing process, and if left intact, could lead to a brittle fracture of the stent struts. A solution of seventy percent methanol and thirty percent nitric acid at a temperature of minus forty degrees centigrade or less has been shown to work effectively as an electropolishing solution. Electrical parameters of the electropolishing are selected to remove approximately 0.00127 cm of material from the surfaces of the struts. The clean, electropolished surface is the final desired surface for attachment to the graft materials. This surface has been found to import good corrosion resistance, fatigue resistance, and wear resistance.
• The graft material orcomponent400, as illustrated inFIG. 1, may be made from any number of suitable biocompatible materials, including woven, knitted, sutured, extruded, or cast materials comprising polyester, polytetrafluoroethylene, silicones, urethanes, and ultralight weight polyethylene, such as that commercially available under the trade designation SPECTRA™. The materials may be porous or nonporous. Exemplary materials include a woven polyester fabric made from DACRON™ or other suitable PET-type polymers.
• In one exemplary embodiment, the fabric for the graft material is a forty denier (denier is defined in grams of nine thousand meters of a filament or yarn), twenty-seven filament polyester yarn, having about seventy to one-hundred end yarns per cm per face and thirty-two to forty-six pick yarns per cm face. At this weave density, the graft material is relatively impermeable to blood flow through the wall, but is relatively thin, ranging between 0.08 and 0.12 mm in wall thickness.
• Thegraft component400 is a single lumen tube and preferably has a taper and flared portion woven directly from the loom, as illustrated for theendovascular graft10 shown inFIG. 1.
• Prior to attachment of thegraft component400 to thestents100,200,300, crimps are formed between the stent positions by placing the graft material on a shaped mandrel and thermally forming indentations in the surface. In the exemplary embodiment illustrated inFIGS. 1 and 4, thecrimps402 in thegraft400 are about two mm long and 0.5 mm deep. With these dimensions, theendovascular graft10 can bend and flex while maintaining an open lumen. Also, prior to attachment of thegraft component400 to thestents100,200300, the graft material is cut in a shape to mate with the end of each end stent.
• As stated above, each of thestent segments100,200 and300 is attached to thegraft material400. Thegraft material400 may be attached to thestent segments100,200,300 in any number of suitable ways. In one exemplary embodiment, thegraft material400 may be attached to thestent segments100,200,300 by sutures.
• The method of suturing stents in place is important for minimizing the relative motion or rubbing between the stent struts and the graft material. Because of the pulsatile motion of the vasculature and therefore the graft system, it is possible for relative motion to occur, particularly in areas where the graft system is in a bend, or if there are residual folds in the graft material, due to being constrained by the aorta or iliac arteries.
• Ideally, each strut of each stent segment is secured to the graft material by sutures. In an exemplary embodiment, the suture material is blanket stitched to the stent segments at numerous points to securely fasten the graft material to the stent segments. As stated above, a secure hold is desirable in preventing relative motion in an environment in which the graft system experiences dynamic motion arising from pulsatile blood pressure, in addition to pulsation of the arteries that are in direct mechanical contact with the graft system. The stents nearest the aortic and iliac ends of the graft system (the uppermostfirst stent segment100 and thethird stent segment300 respectively) are subject to the pulsatile motion arising from direct internal contact. These struts in particular should be well secured to the graft material. As illustrated inFIG. 4, thestitches404 on the upper mostfirst stent segment100 are positioned along the entire zigzag arrangement of struts. The upper and lower apexes of the third stent segment may be stitched utilizing a similar configuration. It is difficult to manipulate the suture thread precisely around the struts that are located some distance away from an open end, accordingly, various other simpler stitches may be utilized on these struts, or no stitches may be utilized in these areas.
• As illustrated inFIG. 4, each of the struts in thefirst stent segment100 is secured to thegraft material400 which has been cut to match the shape of thestent segment100. Theblanket stitching404 completely encircles the strut and bites into thegraft material400. Preferably, thestitch404 encircles the strut at approximately five equally spaced locations. Each of the struts on each end of thethird stent segment300 is attached to the graft material, which has been cut to make the shape of thestent segment300, in the same manner as thefirst stent segment100.
• A significant portion of the graft will not rest directly against vascular tissue. This portion of the graft will be within the dilated aneurysm itself. Therefore, this portion of the graft will not experience any significant pulsatile motion. For this reason, it is not necessary to secure the stent segments to the graft material as aggressively as the stent structure described above. Therefore, only pointstitches406 are necessary for securing these stents.
• It is important to note that a wide variety of sutures are available. It is equally important to note that there are a number of alternative means for attaching the graft material to the stent, including welding, gluing and chemical bonding.
• As stated above, In percutaneous procedures, size is a critical factor. One of the more significant determinants of the final diameter of the catheter system is the bulkiness of the graft material comprising the stent-graft. Accordingly, it is generally accepted that the highest impact on delivery catheter diameter may be achieved by fabricating stent-grafts having thinner walls.
• Typical stent-grafts are fabricated from a woven polyester and are approximately 0.005 inches thick. For example, a stent-graft fabricated from a woven polyester low twist, forty denier, twenty-seven filament yarn having two-hundred thirty yarn ends per inch and one hundred yarn picks per inch, results in a graft material having a wall thickness of approximately 0.005 inches. The graft material is then attached to the inside or outside of a stent or multiple stent segments as described above. Appreciable gains may be achieved in having a graft material thickness in the range from about 0.002 inches to about 0.003 inches.
• For a woven graft, as described above, the wall thickness is determined primarily by weave density and yarn thickness or bulkiness. It is desirable to have a graft which is packed tight enough to prevent significant blood seepage, but not so tight that the yarn bundles pile up on each other. The weaving parameters described above result in just such a graft for the particular yarn described. At this density, the graft material is about as thin walled as it can be without significant permeability. Also, the yarn described above is only lightly twisted, so as the yarn bundles cross over one another, they tend to flatten out. Higher twisting would both make the graft more permeable and thicker, and the yarn bundle would tend to remain cylindrical at the crossover points. The only remaining parameter that can be utilized to thin the graft is smaller yarn bundles.
• There are two variables which influence yarn bundle size; namely, the number of filaments per bundle, and the size or weight of each individual filament. The forty denier, twenty-seven filament polyester yarn described above has a relatively small filament size and a relatively low number of filaments. However, in theory, a much smaller yarn bundle could be contemplated with either few filaments, smaller filaments, or both. For example, a twenty denier yarn bundle could be made from fourteen filaments of the same diameter as described above. If this yarn were woven into a graft material with an appropriately dense weave, one would expect a graft material having a thickness of approximately 0.0025 inches. While this may work as an acceptable graft, it is possible that the long-term integrity of such a graft may not be acceptable due to the forces described above.
• The graft material may be formed utilizing any number of techniques, including weaving, knitting and braiding. Weaving involves the interlacing, at right angles, of two systems of threads known as warp and filling. Warp threads run lengthwise in a woven fabric and filling threads run cross-wise. Knitting is the process of making fabric by interlocking a series of loops of one or more threads. Braiding involves crossing diagonally and lengthwise several threads of any of the major textile fibers to obtain a certain width effect, pattern or style.
• A growing concern with a number of endovascular graft systems has been that over time, holes may develop in the stent-graft wall, which can lead to blood leakage and possible aneurysm rupture. There is only a limited understanding of the mechanism of hole formation; however, it is generally believed to be related to what has been termed chronic micro-motion between the metallic stent support structures and the graft material. Eventually, this micro-motion may cause the graft material to wear away, thereby creating holes.
• One potential way in which to overcome this problem is by more tightly binding the graft material to the stent in areas exhibiting the highest possibility of micro-motion. There are numerous ways by which the graft material may be attached to the stent, for example, polymeric sutures. Accordingly, it may be possible to simply create a thinner polyester graft material as described above, more tightly secure it to the stent in areas which exhibit the greatest potential for micro-motion, and have a lower profile, longer wear resistant stent-graft. However, it would also be beneficial to consider alternate materials for fabricating a significantly thinner graft material with high wear resistance. Higher strength and/or tougher materials may yield a much thinner stent-graft conduit without sacrificing long-term integrity. In fact, some of the materials that may be utilized are so much stronger and tougher than Dacron® polyester, that a significantly thinner stent-graft constructed of these materials may be substantially stronger and more wear resistant than currently available stent-grafts.
• There are a number of new, higher performance fibers that are significantly stronger and tougher than polyester, and which are also biocompatible. Whereas, Dacron® polyester has a tenacity of approximately nine grams per denier, many high performance fibers have tenacities in the range from about thirty-five to about forty-five grams per denier. The more preferred fibers from a strength standpoint for consideration for use in an ultra thin walled stent-graft material, approximately, 0.002 to 0.003 inches include polyaramid, polyphynelenebenzobisoxazole, liquid crystal polymer and ultra high molecular weight polyethylene. From a purely strength standpoint, all of these materials are suitable for ultra-thin walled stent-graft applications. However, from a biostability standpoint, ultra high molecular weight polyethylene fibers may offer a slight advantage in the fact that their basic chemistry is polyethylene, which is known to be relatively inert in biological applications.
• Another important consideration for the above-described fibers is their availability in fine denier yarns. With current stent-grafts fabricated from a forty denier polymer yarn, it would be difficult to fabricate a stent-graft having thinner walls unless the yarn is of a finer denier. A liquid crystal polymer sold under the tradename Vectran is available as a twenty-five denier yarn. A ultra high molecular weight polyethylene sold under the tradename Spectra is available as a thirty-denier yarn. Another ultra high molecular weight polyethylene sold under the tradename Dyneema is available as a twenty to twenty-five denier yarn. It is also important to consider that ultra high molecular weight polyethylene fibers only have a density of 0.97 versus 1.38, so that the same denier yarn would be bulkier in ultra high molecular weight polyethylene, however, due to the substantial improvement in tensile and abrasive properties, much less ultra high molecular weight polyethylene would be necessary to obtain equivalent material properties.
• Polyethylene is a long chain organic polymer formed by the polymerization of ethylene. When formed under low pressure, it will form long polymer chains which increases its resistance to fracture. Ultra high molecular weight polyethylene typically has between six and twelve million ethylene units per molecule. Ultra high molecular weight polyethylene has a low coefficient of friction, a high molecular weight and a high density. Accordingly, a fabric made from ultra high molecular weight polyethylene is highly abrasion resistant, highly impact resistant, and highly resistant to damage by water, salt or fresh. Ultra high molecular weight polyethylene monofilaments have a high tensile strength with the associated advantage of stretch resistance and elasticity. These properties make it especially suitable for tortuous body passageways.
• As stated above, polyethylene has a long documented history of biocompatability. Given this level of biocompatability, coupled with its physical attributes, ultra high molecular weight polyethylene is the preferred yarn for use as a graft material. The ultra high molecular weight polyethylene yarn may be woven, knitted or braided to form the graft material and attached to the one or more stent segments as described above. The graft material may also be used as a stand-alone device for surgical applications or combined with the one or more stents for endovascular delivery.
• In alternate exemplary embodiments, the ultra high molecular weight polyethylene yarn may be blended with a dissimilar material, for example, Dacron® polyester, to manufacture a graft material with altered bulk properties; e.g., stretch potential, while retaining strength and abrasion resistance. In yet other alternate exemplary embodiment, the monofilament of ultra high molecular weight polyethylene may be blended together with another material to attain a true blended yarn such that a fiber or monofilament of one material can be placed next to a monofilament of a second material (third, fourth . . . ) to create a resultant yarn which possesses properties that differ from each of its monofilaments.
• In an alternate exemplary embodiment, silk may be utilized in the construction of ultra-thin walled stent-grafts. In general, silk strands exhibit high strength, toughness and elasticity. By weight, silk is stronger than steel.
• Spiders produce a number of silks for different functions and are therefore useful organisms to produce a variety of structural proteins. The structural fibers of the golden orb-weaver spider (Nephila clavipes), are extremely strong and flexible, and are able to absorb impact energy from flying insects without breaking. Dragline silk fibers dissipate energy over a broad area and balance stiffness, strength and extensibility. Spider dragline silk exhibits a combination of strength and toughness unmatched by high performance synthetic fibers. Dragline silk is the fiber from which spiders make the scaffolding of their webs. It is estimated to be at least five times as strong as steel, twice as elastic as nylon, waterproof and stretchable. In addition, silk proteins have very low anti-genicity. Therefore, silk fibers are well suited for lightweight, high performance fiber, composite and medical applications. The composition of these proteins is mainly glycine, alanine, and other short side chain amino acids, which form anti-parallel beta-pleated sheets by hydrogen bonding and hydrophobic interactions; Lucas et al., Discovery 25:19 1964. Many spider silks are resistant to digestion by proteolytic enzymes; Tillinghast and Kavanaugh, Journal of Zoology 202:212 1977, and insoluble in dilute acids and bases; Mello et al., American Chemical Society Symposium Series 544, Silk Polymers: Materials, Science and Biotechnology pp 67-79, 1995.
• Spider silks have been demonstrated to have several desirable characteristics. The orb-weaver spiders can produce silk from six different types of glands. Each of the six fibers has different mechanical properties. However, they all have several features in common. They are composed substantially of protein, they undergo a transition from a soluble to an insoluble form that is virtually irreversible and they are composed of amino acids dominated by alanine, serine and glycine and have substantial quantities of other amino acids, such as glutamine, tyrosine, leucine and valine. The spider dragline silk fiber has been proposed to consist of pseudocrystaline regions of anti-parallel, β-sheet structure interspersed with elastic amorphous segments.
• Spider silks are an ideal system for exploiting the relationship between protein design and function. While silk production has evolved multiple times within arthropods, silk use is most highly developed in spiders. Spiders are unique both in their dependence on and ability to spin an array of silk proteins throughout their lifetimes. Each type of silk is secreted and stored by a different type of abdominal gland until extruded by tiny spigots on the spinnerets. These proteins are used singly or in combinations for draglines, retreats, egg sacs, or prey-catching snares. Given these specialized ecological roles, individual silks appear to have mechanical properties that correspond to their individual functions.
• Most molecular and structural investigations on spider silks have focused on dragline silk and its extreme toughness (e.g. Xu & Lewis,Proc. Natl. Acad. Sci., USA 87 7120-7124,1990; Hinman & Lewis,J. Biol. Chem.267, 19320-19324, 1992; Thiel et al.,Biopolymers34, 1089-1097, 1994; Simmons et al.,Science271, 84-87,1996; Kummerlen et al.,Macromol.29, 2920-2928, 1996; and Osaki,Nature384, 419, 1996). Dragline silk, often referred to as major ampullate silk because it is produced in the major ampullate glands, has a tensile strength (5×10⁹Nm⁻²) similar to Kevlar (4×10⁹Nm⁻²) (Gosline et al.,Endeavour10, 37-43, 1986; Stauffer et al.,J. Arachnol.22, 5-11, 1994). In addition to this exceptional strength, dragline silk also exhibits approximately 35 percent elasticity (Gosline et al.,Endeavour10, 37-43, 1986).
• The mass production of the dragline silk from spiders is not practical because only small amounts are available from each spider. Furthermore, multiple forms of spider silks are produced simultaneously by any given spider. The resulting mixture has less application than a single isolated silk because the different spider silk proteins have different properties and are not easily separated. In addition, spiders are highly territorial animals, difficult to confine in so-called “spider farms” for silk production.
• There are a number of ways in which spider dragline silk may be mass-produced. For example, by utilizing molecular recombination techniques, one may introduce foreign genes or artificially synthesized DNA fragments into different host organisms for the purpose of expressing desired protein products in commercially useful quantities. Such methods usually involve joining appropriate fragments of DNA to a vector molecule, which is then introduced into a recipient organism by transformation. Transformants are selected using a selectable marker on the vector, or by a genetic or biochemical screen to identify the cloned fragment. In other words, one may clone a specific gene and insert it into bacteria, such asEscherichia coil. The bacteria then manufacture the desired protein. Spider dragline silk, for example, could be made in the laboratory by taking a spider's genes and inserting them into bacteria to produce that spider's strong, durable fiber. The bacteria reproduce and eventually form a cloned colony that produces the synthetic polymer by way of protein synthesis.
• Other techniques for creating commercially acceptable quantities of dragline spider silk may involve transplanting the glands from select spiders into different hosts that may produce the silk or silk proteins in vast quantities.
• Regardless of the method of creation or manufacture, spider dragline silk may be utilized in the construction of ultra-thin walled stent-grafts or stand-alone grafts. Spider dragline silk is biocompatible, lubricious and recognized as one of the strongest fibers known to man. The dragline silk may be utilized on its own or blended with other fibers, including those discussed herein, to modify the bulk material properties of the completed graft. The graft may be utilized alone or in conjunction with a metallic stent as described herein. The dragline spider silk may be woven, braided or knitted to modify the bulk properties of the completed graft. The graft may be utilized in cardiac, neuro, or peripheral vascular applications.
• As stated above, spider dragline silk may be utilized alone or in combination with other fibers to create an ultra-thin graft material. The spider dragline silk material for the graft may be manufactured from natural or man-made spider dragline silk fibers.
• The spider dragline silk material may be attached to the stent structure using any number of ways, including those described herein. Alternately, the spider dragline silk material may be utilized as a stand-alone device500 as illustrated inFIG. 5. In the exemplary embodiment illustrated inFIG. 5, the spider dragline silk fibers are woven. The substantially tubular graft may be positioned surgically or utilizing percutaneous methods.
• Although shown and described is what is believed to be the most practical and preferred embodiments, it is apparent that departures from specific designs and methods described and shown will suggest themselves to those skilled in the art and may be used without departing from the spirit and scope of the invention. The present invention is not restricted to the particular constructions described and illustrated, but should be constructed to cohere with all modifications that may fall within the scope for the appended claims.

Claims (8)

1. An endovascular graft comprising:

one or more scaffold structures;

a biocompatible, high tensile strength, abrasion resistant, highly durable thin-walled graft material affixed to the one or more scaffold structures, the graft material including spider dragline silk; and

at least one connector for connecting the graft material to the one or more scaffold structures.

2. The endovascular graft according toclaim 1, wherein the one or more scaffold structures comprises a plurality of first stents, a second stent and a third stent.

3. The endovascular graft according toclaim 2, wherein the graft material is affixed to an outer portion of the plurality of first stents, the second stent and the third stent.

4. The endovascular graft according toclaim 1, wherein the graft material comprises a plurality of crimps between the one or more scaffold structures.

5. The endovascular graft according toclaim 1, wherein the graft material is woven.

6. The endovascular graft according toclaim 1, wherein the graft material is knitted.

7. The endovascular graft according toclaim 1, wherein the graft material is braided.

8. The endovascular graft according toclaim 1, wherein the at least one connector comprises sutures.

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